It is experimentally demonstrated that an a-Si : H photodiode with ninip stacked structure (reach-through structure) can detect infrared light of 1.31 μm and 1.55 μm. A maximum gain-quantum efficiency product of 0.58 is obtained at 1.31 μm. This value of gain-quantum efficiency product is comparable to the quantum efficiency of a non-gain-enhanced a-Si : H pin photodiode at visible wavelengths. The response speed of the photodiode is slow, on the order of ms. An insertion of a carrier-injection-blocking layer between a-Si : H and the electrode improves the response speed, but decreses the gain-quantum efficiency product.
An amorphous Avalanche Photo-Diode (APD) with a heterojunction of a-SiC : H and a-SiGe : H was formed. The bandgaps of a-SiC : H and a-SiGe : H are 3.5eV and 1.55eV, respectively. The heterojunction has larger conduction band discontinuity than the bandgap of a-SiGe : H. In this amorphous APD, the photo-current multiplication is observed under low electric field. The quantum efficiency strarts to exceed unity when the conduction band discontinuity becomes larger than the bandgap of lower-gap material, and it is likely to saturate at two. The slope of photo-electric conversion characteristics is 1.00. This multiplication is explained by the impact ionization process at the band edge discontinuity region.
The photocurrent multiplication has been observed in a staircase photodiode having three conduction-band discontinuous regions. Using this photodiode, we observed the photocurrent multiplication gain of 6. It is found that the dominant mechanism of the multiplication is based upon the electronic ionization effect at the conduction based discontinuous edge in the conduction band.
By connecting an avalanche multiplier film to a CMOS readout circuit via micro-bump electrodes, a newly sensitive solid-state imager has been developed. Optimization of the process conditions for indium bump electrode made it possible for micro-bump of 4 μm width and 5μm height to be formed into a 2/3-inch matrix array of 380k pixels. A prototype imager was constructed with a 0.5-μm-thick avalanche photoconductive film. Clear avalanche multiplication of about ten times was cbserved at an applied voltage of 75V. The imager has a good resolution and no recognizable after-images.
An improved version of the Super-HARP pickup tube has been developed by increasing the thickness of the target. Called the New Super-HARP tube, it is 8 times more sensitive than the Super-HARP tube and the thicker target also helps to improve lag characteristics. Moreover, a special target structure is introduced to prevent spurious images. A handheld color camera, which uses the New Super-HARP pickup tubes, has been shown to have outstanding characteristics, including ultrahigh sensitivity, low lag and high resolution. The New Super-HARP camera is expected to become a powerful tool in nightime emergency news gathering and the production of science programs.